Development of miniaturized microfluidic systems has resulted in the ever increasing use of microanalytical devices that are able to perform a multitude of chemical, physical, and/or electrical processes on a microscale. Applications for such devices include a wide variety of tasks requiring chemical, physical, or electrochemical analysis and testing, including analytical chemistry, biochemistry, medical testing and instrumentation, and industrial process control. In order to effectively use and control many such devices, there is a need for the conduction of electricity between various components, or between a fluid and a nearby component. It is important that a circuit be completed without an unnecessary increase in resistance. Thus one of the requirements for a suitable conductor in such microcircuits is that it provides a lower resistance than other components in a particular electrical circuit.
As generally described herein, the term “microfluidic” refers to a system or device having channels and chambers that may be fabricated on the micron or submicron scale, i.e., having at least one cross-sectional dimension in the range from about 0.1 μm to about 500 μm. Various methods for fabrication of devices having such characteristics are known to those of ordinary skill in the art and to whom this specification is addressed. However, the use of such devices wherein controlled electrokinetic transport is utilized for various functions has been described by Ramsey in U.S. Pat. No. 5,858,195, issued Jan. 12, 1999 and entitled APPARATUS AND METHOD FOR PERFORMING MICROFLUIDIC MANIPULATIONS FOR CHEMICAL ANALYSIS AND SYNTHESIS, the disclosure of which is incorporated herein in its entirety by this reference. The reader is referred thereto for additional background with respect to electrokinetic material transport. In any event, it should be noted that many heretofore known features incorporated into microfluidic devices may be fabricated using standard photolithographic, wet chemical etching, and bonding techniques. Some aspects of such techniques may also be used to assist in the manufacture and incorporation into such devices the polymer salt bridges described herein.
In microfluidic devices, to provide the function of a miniature chemical factory, the device needs a way to move fluids, such as via pressure from pumps, or by electroosmotic flow. When electrical potential is used, either for moving fluids or for analytical purposes, the device must be provided with components and structures for moving electrical current between desired locations. Metal electrodes are generally undesirable or impractical in microfluidic devices because of localized electrical field inhomogenities and possible electrochemical reactions. Thus, in microfluidic devices, it would be advantageous to provide electrically conductive but fluid flow resistive structures that can be easily installed at any desired location in or adjacent to a selected fluid transport or fluid containing microchannel. In order to provide such a device, it would be advantageous if such electrically conductive but fluid flow resistive devices could be integrally manufactured within the microfluidic channels themselves. Thus, the important advantages of a novel, cast-in-place, monolithic polymer salt bridge, with formulatable and selectable conductivity characteristics, as well as compatibility with a particular aqueous or organic solvent mixture, or with selected analytic reagents, can be readily appreciated.